Effect of pressure due to change in diameter of pipe

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Nitheeswar
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The pump has an outlet size of 4 inch
1) In case 1, 4 inch pipe is connected at length of 20 m
2) In case 2, 2 inch pipe is connected using reducer
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
What happens to pressure at the end of the pipe in case 2?
 
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The pump has an outlet size of 4 inch
1) In case 1, 4 inch pipe is connected at length of 20 m
2) In case 2, 2 inch pipe is connected using reducer
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
What happens to pressure at the end of the pipe in case 2?
 
Nitheeswar said:
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
False.

All other things the same between the two systems:

If system2 is 20 ft. of 2 inch pipe (horizontal) and you want flow ##Q## out of it, it requires a certain pressure at the pump discharge ##P_2##.

If system1 is 20 ft. of 4 inch pipe (horizontal) and you want flow ##Q## out of it, it requires a certain pressure at the pump discharge ##P_1##, where ##P_1< P_2##.

In other words you need an effectively different pumps to achieve the flow rate in each system. Meaning impeller changes, speed changes, etc...

If you are running a positive displacement pump( aka constant volume pump) the flow rate will be equal, but not at the same pressure. The pressure adjust to whatever the systems demands at a flow with those pumps. Their pump curve is a vertical line at a fixed speed.
 
Last edited:
Nitheeswar said:
The pump has an outlet size of 4 inch
1) In case 1, 4 inch pipe is connected at length of 20 m
2) In case 2, 2 inch pipe is connected using reducer
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
What happens to pressure at the end of the pipe in case 2?
Isn't it one out of two: either the flow rate is the same OR the pressure is the same ?

And: doesn't it matter what's at the end of the pipe ? Atmospheric pressure, something else ?

##\ ##
 
Is this pressure difference only due to contraction and head loss in 2 inch (compared to 3 inch)?
 
Is this pressure difference only due to contraction and head loss in 2 inch (compared to 4 inch)?
 
Nitheeswar said:
Is this pressure difference only due to contraction and head loss in 2 inch (compared to 4 inch)?
Each piping system has its own pressure vs discharge relationship. They are not independently adjustable. It would also have to do with kinetic head consideration, not just head loss.